The present invention relates to photoelectric smoke detectors and particularly those of the reflected-light type. The invention is particularly concerned with means for testing the sensitivity of such smoke detectors.
A reflecting-type photoelectric smoke detector typically includes a dark smoke chamber into which light is directed from a light source. This light is reflected by smoke particles in the smoke chamber to a light responsive cell such as a photodiode for actuating an alarm when the smoke density in the smoke chamber and, therefore, the amount of light reflected to the photodiode, exceeds a predetermined level.
Since the purpose of a smoke detector is to be immediately activated in the event of a fire at any time of the day or night, it is necessary that some means be provided for testing how well the smoke detector is functioning in the field. In particular, it is necessary to be able to test the sensitivity of the device, i.e., the smoke density at which the detector alarm will be actuated. Since it is normally impractical to expose a smoke detector in the field to an increasing amount of smoke, and even more impractical to quantify the amount of smoke to which the detector is being exposed, the next best thing is to closely simulate the presence of smoke in the smoke chamber to determine the level at which the alarm will be actuated.
A number of techniques have heretofore been used to test the sensitivity of photoelectric smoke detectors. One such technique, disclosed in U.S. Pat. No. 4,053,785 involves means for gradually increasing the amount of light from an auxiliary light source which is allowed to reach the photodiode to determine the point at which the alarm will be actuated. But such an approach does not make the pertinent evaluation, i.e., how the system will respond to the primary light source which is used during normal operation. Thus, such a system may test perfectly even though it has a defective primary light source.
Another technique is illustrated in U.S. Pat. No. 4,099,178 which involves the use of an alternate light path to the photodiode for test purposes. The system in that patent has a normally-closed test light path which does not pass through the smoke chamber, and which can be opened for test purposes. The system also provides means for adjusting the amount of light which is allowed to pass along the test path. But in the device of the '178 patent the light path passes through a very small aperture. Thus, very little light is allowed to reach the photodiode and very minute variations in the amount of that light will have a significant effect. Accordingly, the parts must be made to very exacting tolerances. Furthermore, since the test light path does not pass through the smoke chamber, the system does not test the condition of the smoke chamber. This is significant since spurious obscuring bodies, such as spider webs or the like, could be present in the smoke chamber which would falsely tend to indicate the presence of smoke. Such foreign matter cannot be detected with the test apparatus of the '178 patent.
Other prior art techniques involve inserting a test member into the smoke chamber for scattering the light from the light source. Such a system is disclosed in U.S. Pat. No. 3,868,184. But the relative signal from a smoke particle in the scattering region of the smoke chamber depends on its location, and a slight difference in relative signal strength. Thus, systems such as that disclosed in the '184 patent, wherein the test member is inserted in the normal scattering region of the smoke chamber, tend to be relatively inaccurate because it is extremely difficult to precisely place the light-scattering test object in the scattering region and to precisely determine the appropriate size for the test object.